Hi Tomaiah,
He may not have called or texted you because he isn’t ready to take his interaction with you to that level.
For the past five months, he has been staring at you and sparking up conversation. However, you have been very shy and standoffish.
During the course of those five months, he was most likely hoping that you would start flirting back with him.
In other words, he wanted you to also spark up conversation with him and engage with him on a flirtatious level.
However, because you are very shy and standoffish, you didn’t do any of that.
When you sent him that birthday card and wrote that you like him and left your number, it was too much and too unnatural for him.
There was no real buildup that led to what you wrote in that birthday card.
He didn’t get to have a flirtatious back-and-forth with you over the course of those five months in order to determine whether the both of you truly had romantic chemistry.
Thereby, what you wrote in that birthday card probably hit him like a deer in headlights.
Calling or texting you without any romantic or flirtatious buildup would just feel too awkward to him. This is why he has most likely chosen not to call or text you.